Alignment system for optical lithography

ABSTRACT

An alignment system for optical lithography uses cameras fixed to a movable stage and to a lithography unit to view unique microscopic non-uniformities that are inherent to the surface of a work piece, e.g., metal or ceramic microcrystalline grains, for position referencing. Stage cameras image two sites on the work piece through windows in the stage to establish original position templates. After the work piece has been repositioned, e.g., reversed topside-down, the same two sites are again viewed and template matching establishes the transformed coordinates of the work piece, e.g. by a lithography unit camera under which the stage moves to approximate site locations. Two corner cameras can serve as a coarse positioning mechanism. The alignment system is particular useful for backside alignment in printed circuit board lithography.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/240,781 filed Sep. 29, 2008, now U.S. Pat. No. 7,847,938, which inturn claims priority under 35 U.S.C. 119(e) from U.S. provisionalapplication No. 60/997,280, filed Oct. 1, 2007.

TECHNICAL FIELD

The present invention relates to systems and methods for determining theposition of a work piece, such as a printed circuit board, forlithography, and relates especially to reestablishing positioncoordinates of a reversed or repositioned work piece to obtain precisealignment of lithographic features in successive exposures, such as onopposite sides of the work piece.

BACKGROUND ART

For printed circuit boards, it is necessary to print aligned circuitpatterns on both sides of an inner core. The inner cores are composed ofsurface layers of a conductive material, such as copper, on both sidesof a dielectric substrate, with the conductive surface layers beingselectively removed according to patterns established by lithography toform circuit features. These inner cores will eventually get laminatedtogether to form the final printed circuit board. When a lithographyunit exposes each side of the inner core separately, it is important tohave a lateral alignment scheme to match position coordinates on bothsides so that the front and back patterns will be properly aligned toeach other.

The usual alignment method first creates special alignment marks, suchas drill holes or patterned features, on one or both sides of the coreto establish position references. At least two such marks are needed toaccount for both lateral x-y displacements in the plane of the surfaceand rotational skew about any axial point in the plane of the surface.Cameras or physical sensors locate such special marks to determine theposition of the work piece.

One example of such an alignment system is found in U.S. Pat. No.6,701,197 (Ben-Ezra et al.), which writes an alignment pattern on oneside of a printed circuit board, while a first lithographic image iswritten on the other side of the board. The alignment pattern is viewedwhen the board is flipped over. A second lithographic image is thenwritten on the same side as the alignment pattern. Knowledge of thealignment pattern location defines the position of the already writtenfirst image that is now on the underside, so the second image alignswith the first. Other examples of backside alignment systems forlithography that use alignment marks formed on the substrate are foundin U.S. Pat. No. 6,525,805 (Heinle), U.S. Pat. No. 6,861,186 (Pagette etal.), and U.S. Pat. No. 6,936,385 (Lof et al.).

It is desirable in many cases to avoid creating any holes in thesubstrate, so this is a less than adequate alignment solution. Formationof special alignment marks on the substrate is better, but also is notalways desired. Additionally, both techniques have accuracy issues. Theaccuracy of alignment marks is limited by the size of the marksthemselves, and finer control of alignment is sought.

SUMMARY DISCLOSURE

Better alignment accuracy is achieved without having to form any holesor special alignment marks by a method of aligning a work piece thatuses unique inherent microscopic characteristics of the work piecesurface, such as the grain structure of metals or ceramics, as aposition reference. Two widely spaced apart surface sites are chosen formicroscopic examination by cameras fixed to a movable stage onto whichthe work piece is placed, and also by a camera fixed to the lithographyunit. Pattern matching of the two site images first obtained by thestage cameras with images of the same sites obtained by the lithographyunit camera when the work piece is reversed (or otherwise repositioned)allows the relative positions of the work piece to be accuratelycomputed and thus the successive printed lithography features, such asfront and backside features, to be aligned. Image pixels correspondingto surface areas on the order of just 1-micron square or smaller allow acorresponding positioning accuracy to be achieved.

An exemplary alignment system of the present invention may comprisefirst and second stage cameras that are fixed to a movable stage onwhich the work piece is positioned and that view respective first andsecond sites of the work piece surface through windows in the stage.Another camera that is fixed to the lithography unit is used, as needed,to view the same sites from above whenever the work piece is reversed.Some coarse locating mechanism, such as other cameras fixed to the stageand lithography unit, may be provided to enable approximate location ofthe sites when the work piece is reversed or otherwise repositioned. Acomputer is adapted to perform a pattern match of the first set ofmicroscopic grain images obtained by the stage cameras from the firstand second sites on the work piece surface with a second set ofmicroscopic grain images obtained from the same sites after the workpiece has been reversed or repositioned, thereby computing relativeposition coordinates. The computer then controls fine movement of thestage under the lithography unit based on the computed positioncoordinates for the work piece. All of the cameras involved in thealignment have accurately known positions relative to the stage orlithography unit through an initial calibration routine.

An alignment method in accord with the invention thus comprises placinga work piece onto the movable stage, viewing and storing microscopicgrain images from the first and second surface sites on a first side ofthe work piece using the stage cameras, and exposing a second side ofthe work piece to a lithographic writing process by the lithographyunit. Then, after the work piece has been repositioned on the stage,again viewing and storing microscopic grain images at approximatelocations of the first and second sites on the first side of the workpiece (moving the stage and using the lithography camera in the casewhere the work piece has been reversed), performing a pattern match ofthe respective sets of images in order to compute a relative positionand rotation of the repositioned work piece, and again exposing the workpiece to a lithographic writing process by the lithography unit, usingthe computed position to achieve alignment of the first and secondlithographic exposures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lithography apparatus with movablestage, lithography unit, and the alignment system in accord with thepresent invention.

FIG. 2 is a top view of the movable stage in FIG. 1, showing windows forunderlying stage cameras of the alignment system in accord with theinvention. Dashed perimeter lines outline the potential position of awork piece to be placed on the stage.

FIG. 3 is a side view of the lithography apparatus of FIG. 1.

FIG. 4 is an enlarged sectional view of the movable stage with a windowand underlying stage camera.

FIG. 5 is a flow diagram of a lithographic process embodiment containingan alignment method in accord with the present invention.

FIG. 6 is an exemplary corner image.

FIG. 7 is an exemplary site image with overlay of matching template.

DETAILED DESCRIPTION

The invention relates predominantly to two-sided optical lithography forprinting patterns successively on both sides of a printed circuit board.More generally, the work piece may be any substrate where alignedpatterning may be required, including printed circuit boards, ceramichybrid circuit substrates, semiconductor substrates, semiconductorpackages, flat panel displays, etc. Additionally, while front-to-backalignment of lithographic features on reversible substrates is the chiefmotivation, the invention can also be applied to any lithographic methodwhere position coordinates of a work piece must be reestablished forproperly aligning successive exposures; as when the work piece isremoved after a first exposure for cleaning or other processing, thenrepositioned under the lithography unit for another exposure, possiblywith a different mask.

With reference to FIGS. 1 to 3, a lithography apparatus 10 generallyincludes a movable stage, table or chuck 12 onto which a work piece 14may be placed, such as an inner core of a printed circuit board. Alithography unit 16 is situated above the stage so as to expose the workpiece to a lithographic writing process. The stage 12 is typicallymovable at least two dimensions, such as in either direction along theorthogonal axes X and Y. There may also be a vertical Z component ofmotion for achieving desired scale of the exposure. The movable stagemay include a number of mechanical stops 18 to limit the placement ofthe work piece 14 at one or more of its edges. The lithography unit 16may be fixed in place in the overall apparatus 10, while the stage 12and work piece 14 move under it, or the unit 16 may itself be movable inone or more X, Y or Z directions. One such lithography apparatus 10 towhich the alignment system of the present invention may be applied isdescribed in further detail in U.S. Pat. Nos. 7,167,296 and 7,295,362assigned to the assignee of the present invention and incorporated byreference herein. It uses direct writing by light projection using aspatial light modulator that contains an array of individuallyswitchable elements to regulate the dose of optical energy received bythe photosensitive substrate at each pixel location. Other lithographyapparatus could also be used.

The optical alignment system includes a pair of upward-looking cameras20 and 22 that are fixed to a movable stage 12 on which the work piece14 is positioned and that view respective first and second sites of thework piece surface through respective first and second windows 24 and 26in the stage 12. That is, the first-site stage camera 20 looks upthrough window 24 at a first site on the work piece 14, and thesecond-site stage camera 22 looks up through window 26 at a second siteon the work piece 14. The windows 24 and 26 protect the cameras 20 and22 underneath from dirt. The two sites which are defined by the cameralocations are to be widely spaced apart for best alignment resolution,but are otherwise arbitrary and the exact locations on the work piece 14are determined within those constraints by the particular manner inwhich the work piece 14 is first placed on the stage 12. The opticalalignment system likewise includes a downward-looking lithography unitcamera 28 fixed to the lithography unit 16 to view the same first andsecond sites when the work piece is reversed (bottom side up). In thatcase, the stage 12 moves laterally in two dimensions X and Y under thelithography unit 16 and its camera 28 until the sites are approximatelylocated.

All three of these site-viewing cameras 20, 22 and 28 image non-uniformmicroscopic characteristics that are inherent to the work piece surfaceand unique to each location, such as the grain structure of metals orceramics. For example, printed circuit board cores typically haveconductive surfaces made of copper, but other metals might also be used.The graininess of the copper or other metal provides excellent uniquefeatures for use in lateral alignment. Ceramic or composite surfacessubject to lithographic patterning, such as for ceramic hybrid circuitsubstrates and semiconductor packages, also have a discernible inherentmicroscopic grain structure. Other polycrystalline materials, such as insemiconductors used in some arrays of photovoltaic cells and the like,also have inherent grain structures that could be exploited forlithographic alignment.

The system requires images of the surface with sufficient magnificationthat the grain in the material is visible. Because of the randomness ofthe position, size and orientation of the grains, each location producesimages that are unique enough to serve as a precise position reference.A first set of microscopic grain images are obtained by the stagecameras 20 and 22 from the first and second sites on the work piecesurface. To further improve lateral registration of positions,sub-images that contain the most unique grain patterns are extractedfrom the overall images acquired by the cameras 20 and 22 to be storedas image templates for pattern matching. For example, the camera imagearea may be 160 by 160 pixels, 240 by 240 pixels or larger, while theselected template sub-images may be 80 by 80 pixels or smaller, whereeach image pixel represents an area on the work piece surface on theorder of 1 micron-square.

In the case where the work piece 14 is to be removed and subsequentlyrepositioned back onto the stage without reversing, the templatesub-images should chosen to be near the center of the images firstacquired by the stage cameras 20 and 22 in order to ensure that thesesame sub-images are again viewable through the windows 24 and 26 by thestage cameras 20 and 22 when the work piece is repositioned. In the casewhere the work piece is to be reversed, this constraint on selecting thesub-images does not apply, since the lithography unit camera 28 is ableto view any portion of the work piece that faces it, including anyportion of the original first and second sites and neighboring areas,simply by moving the stage 12 under the camera 28 to the approximatelocations of those original sites. Thus, the stored sub-images selectedas the positioning templates are readily found again after reversal ofthe work piece 14. In either case, a second set of microscopic grainimages are obtained from the same sites after the work piece has beenreversed or otherwise repositioned and the corresponding sub-images areidentified by template matching using a known cross-correlationalgorithm or similar technique.

For lithography, the surface of the work piece 14 being positioned underthe lithography unit 16 is normally covered with photo-resist to beselectively exposed with some pattern. A surface covered withphoto-resist is most effectively imaged in a dark field mode. Thisavoids light being reflected from the photo-resist surface into theimage and obscuring the pattern to be sought. Dark field imaging alsoavoids specular reflection from the windows 24 and 26 into the stagecameras 20 and 22. As represented in FIG. 4 for camera 20, a suitablelight source for this imaging mode is an annular arrangement of lightemitting diodes (LEDs) 27 around the optic axis of each camera 20, 22,and 28, with a diffuser 29 in front of the LEDs 27 to ensure a trulyannular illumination (and not a collection of point sources). Thephoto-resist should be substantially insensitive to the cameraillumination. Red light has been found to be effective. Typically, thewindows will have an area of about 30 mm square and a thickness of about2 mm. Sapphire or glass windows can be used. The underside of the stageshould be beveled at the window perimeters to allow light that isdirected inwardly at angle from the ring of LEDs 27 to be incident uponthe window and overlying work piece 14.

Since the sites viewed by the cameras 20, 22, and 28 are not more than afew hundred microns wide and the target template areas are even smaller,some coarse locating mechanism is provided to facilitate locating thesites'after the work piece is repositioned. For example, two cornercameras 30 and 32 fixed respectively to the stage 12 and lithographyunit 16 may be provided to view two corners of the work piece 14, onefrom below and one from above. Cutouts or windows 34 and 36 are made inthe stage 12 over the corner areas to provide visible contrast. Brightfield or dark field images may be used. Camera images of the work piececorners might typically be 640 by 480 pixels, where the pixel size is onthe order of 10 microns square.

For example, an upward-looking corner stage camera 30 mounted on thestage 12 acquires the image of a first corner of the work piece 14.Camera 30 looks at this first corner in dark field illumination througha first corner window 34 in the stage 12 that protects camera 30 fromdirt. A downward-looking camera 32 mounted in fixed relation to thelithography unit 16 acquires the image of a second corner of the workpiece 14. Corner camera 32 looks at this second corner with dark fieldillumination. Behind the second corner there is a cutout 36 so that thecontrast will be very similar to that of corner images acquired frombelow. The cutout 36 may be elongated to accommodate a variety of workpiece lengths. When the work piece 14 is reversed, the first corner willoverlay the cutout 36 that the second corner formerly occupied. The workpiece 14 will also be pushed against the mechanical stops 18 so that thesecond corner will be sure to overlay the window 34 formerly occupied bythe first corner.

From the acquired corner images of the work piece 14, the coarselocations of the two corners are calculated. When the work piece 14 isreversed, the coarse locations of the two corners are recalculated. Anyknown edge recognition algorithm may be used for this purpose. From therelative locations of the corners, a coarse coordinate system(translation and rotation) for the repositioned work piece can becomputed to estimate the approximate locations of the first and secondsites.

To determine the positions of the cameras themselves, the alignmentfurther includes a calibration camera 40 fixed to the movable stage 12under a calibration window 42 in the stage 12. The calibration window 42has a fiducial mark 44 that serves as an absolute position reference forthe stage 12. The calibration camera 40 views the fiducial mark 44 andeach of the lithography unit cameras 28 and 32. A calibration laser 46is fixed to the lithography unit 16 and is operative to produce adownward-directed laser beam that provides a light spot to be viewed inturn by cameras 40, 20, 22, and 30. The position of the calibrationcamera 40 is determined relative to the fiducial mark 44 using a knowncross-finding algorithm applied to the calibration camera's image of themark 44. The laser spot directed onto the calibration window 42, asviewed by the calibration camera 40, fixes the position of the laser 40relative to the fiducial mark 44. From these two established positionsof the calibration camera 40 and the calibration laser 46, the othercamera positions are fixed by applying the laser light spot to be viewedby the stage cameras 20, 22, and 30, and by viewing the lithography unitcameras 28 and 32 relative to the fiducial mark 44 by the calibrationcamera 40. Thus, the calibration camera 40 using the fiducial mark 44locates the downward-looking cameras 28 and 32 on the lithography unit16 and locates the calibration laser 46, while the calibration laser 46using its beam spot locates the upward-looking site and corner stagecameras 20, 22, and 30.

A computer system 50 is provided with the lithography apparatus 10, notonly to control movement of the movable stage 12 and lithographicwriting by the lithography unit 16, but also to perform the variousalignment computations. A cable 52 supplies image data from the stagecameras 20, 22, 30, and 40. Likewise, a cable 54 supplies image datafrom the lithography unit cameras 28 and 32. The alignment computationsperformed by the computer system 50 include the initial positioncalibration of the cameras, edge recognition for the corner images,template selection from the original first-site and second-site imagesby the stage cameras 20 and 22, coarse computation of the approximatesite locations based on comparison of the corner images afterrepositioning with the originals using a known coordinate transformationalgorithm, template matching by cross-correlation of the second set offirst-site and second-site images with the selected template, and finecomputation of the repositioned coordinates (displacement and skew),again with a known coordinate transformation algorithm. These alignmentcomputations are built from known algorithms, such as those provided ina standard library of commercial mathematics software, such as MathLab.

With reference to FIG. 5, a process flow for lithographic patterning ofa work piece, such as a printed circuit board core, and that includesaccurate aligning of the work piece for exposure by a lithography unit,particularly after repositioning or reversal, begins by first loading 60the printed circuit board or other work piece in an obverse position(first side down) onto the movable stage of the lithography apparatus.

Macro images of work piece corners are viewed and captured 62 by thecorner cameras and stored in the computer system. The stage will need tobe moved so that the lithography unit's corner camera is situated abovea corner, thereby also establishing an estimated width of the workpiece. From these images, original coarse positions of the corners canbe calculated using a known edge recognition algorithm. FIG. 6 shows anexemplary corner macro image for coarse positioning.

Micro images of selected first and second sites of the work piece areviewed and captured 64 through stage windows by the stage site camerasand stored in the computer system. From these site images, sub-imagetemplates can be selected by the computer system for later templatematching after the work piece has been reversed or otherwiserepositioned. FIG. 7 shows an exemplary site image with an overlayindicating a selected template to be matched. The approximate locationsof the first and second sites are known from the relation of the stagesite cameras to the calculated corner positions.

Once these corner and site reference images have been captured andstored, the second (face up) side of the work piece is exposed to alithographic writing process 66 by the lithography unit. Depending onthe work piece and the particular lithographic process, the work piecemight undergo some intermediate processing (development, etching,cleaning, etc.) at this point. In any case, the work piece is removedfrom the stage and subsequently repositioned 68 back onto the stage. Inthe case of printed circuit board lithography, the board is typicallyimmediately reversed (the bottom first side up and the top second sidedown) on the movable stage without any intermediate processing. In allcases, the repositioned or reversed work piece is no longer in exactlythe same position on the stage as it was originally, and for alignmentpurposes the position coordinates relative to the original position needto be computed.

Macro images of work piece corners are again viewed and captured 70 bythe corner cameras and stored in the computer system. The stage willagain need to be moved so that the lithography unit's corner camera issituated above a corner; in particular, so that the same corners arerecaptured. In the case of work piece reversal, the first and secondcorners have switched places, so that the opposite camera from theoriginal does the capturing. From these images, new coarse positions ofthe corners are calculated 72, again using a known edge recognitionalgorithm. A coarse coordinate transformation maps the first coordinatesystem based on the original corner locations to an updated coordinatesystem based on the new corner locations. This coarse mappingestablishes the approximate new locations of the original first andsecond sites, taking into account the mirroring of the positioncoordinates caused by reversal of the work piece, as well as thedisplacement and rotation that have likely occurred due therepositioning of the work piece onto the stage.

Micro images of the work piece are viewed and captured 74 at theapproximate locations of the first and second sites. If the work piecehas been reversed, as is typically the case for printed circuit boardprocessing, the lithography unit's site camera is used to view the nowupwardly facing first side of the work piece. The stage is moved underthe lithography unit so that the respective first and second sites canbe successively observed. If the work piece has been simply replacedback in the same original obverse position, then the first and secondsites should again be in the view of the stage site cameras through therespective site windows in the stage. The mechanical stops on the stagewill ensure that repositioned work piece is nearly in the same positionas it was original, while template matching will identify the exactmovement caused by the repositioning.

In either case, template matching between the original selectedtemplates for the first and second sites with the new first and secondsite images will identify the relative image shifts. The templatesub-image portion of the new site images need not be, and are usuallynot, identical to the original sub-images, due to differences incameras, the presence of the window in one image and not in the image ofa reversed work piece, the presence of dust or other artifacts in onebut not the other microscopic image, etc. However, knowncross-correlation routines score each potentially matching sub-imageagainst the original template, and choose the sub-image with the bestscore as that constituting a match. Based on the matching sub-images forthe two sites, the coordinate positions are further updated 76 usingstandard coordinate transformations, establishing the fine relationshipbetween the original and repositioned first and second site positions.

The topside of the work piece is exposed to a lithographic writingprocess 78 by the lithography unit based on the computed relativepositions of the repositioned work piece. In the case of reversal of thework piece, the former first bottom side is now being exposed, whichshould register with the position of the pattern previously formed onthe second side. In the case of simple repositioning, a second patternis formed over the first on the same side of the work piece, and shouldlikewise align. After this second lithographic processing, the workpiece can then be unloaded 80.

1. An optical alignment system, comprising: a coarse locating mechanismmeans for establishing a position of a substrate having an opticallynon-uniform surface; at least one first camera aimed towards theoptically non-uniform surface so as to obtain original microscopic grainimages of multiple regions of the substrate and, after a repositioningof the substrate, to obtain new microscopic grain images of the multipleregions; pattern recognition means for fine position locating of thesubstrate by use of the microscopic grain images obtained by the atleast one camera and for performing a pattern match, after therepositioning of the substrate, between the original microscopic grainimages and the new microscopic grain images; and means for determiningrelative position coordinates of the repositioned substrate based on theperformed pattern match.
 2. The optical alignment system as in claim 1,wherein the substrate is in a position on a stage, the at least onefirst camera that creates the original microscopic grain images viewingthe multiple regions of the substrate through windows in the stage. 3.The optical alignment system as in claim 2, wherein the at least onefirst camera comprises a separate camera for each of the multipleregions of the substrate, each separate camera being in a fixed locationrelative to the stage.
 4. The optical alignment system as in claim 2,wherein the at least one first camera comprises separate first andsecond cameras for the respective first and second sites on the workpiece, each separate camera being in a fixed location relative to thestage.
 5. The optical alignment system as in claim 1, wherein therepositioned substrate is a reversed substrate, the new microscopicgrain images being obtained from at least one other camera located onthe opposite side of the substrate from the at least one first cameraand in known positional relationship to the at least one first camera.6. The optical alignment system as in claim 1, wherein the multipleregions comprise a first and second region on a first side of thesubstrate.
 7. The optical alignment system as in claim 1, furthercomprising a lithography unit configured to record a first patternedimage on the substrate and to subsequently record a second patternedimage on the repositioned substrate in known alignment with the firstpatterned image.
 8. The optical alignment system as in claim 7, whereinthe substrate is a printed circuit board substrate.
 9. An alignmentsystem for locating a position of a reversible work piece, comprising:one or more stage cameras situated to view first and second sites on afirst side of the work piece through a stage so as to obtain microscopicgrain images of the work piece at the respective first and second sites;an area camera situated to view approximate locations of the respectivefirst and second sites on the first side of the work piece when the workpiece is reversed so as to obtain microscopic grain images at therespective approximate locations of the first and second sites; and acomputer adapted to determine a position of a reversed work piece viewedby the area camera relative to the first and second sites viewed by theone or more stage cameras by performing a pattern match of themicroscopic grain images of the work piece surface obtained by the areacamera and by the one or more stage cameras.
 10. The alignment system asin claim 9, further comprising: a pair of corner cameras situated toview a first corner of the work piece both through the stage and whenthe work piece is reversed.
 11. The alignment system as in claim 9,further comprising: a fiducial mark on an alignment window of the stage;an alignment camera situated to view the fiducial mark; and a lasersource in fixed relation to the area camera and operative to provide alaser spot onto the alignment camera.
 12. The alignment system as inclaim 9, wherein the cameras include an annular ring of LEDs aroundtheir aperture for dark field illumination of the work piece.
 13. Theoptical alignment system as in claim 9, further comprising a lithographyunit configured to record a first patterned image on a first side of thework piece and to subsequently record a second patterned image on asecond side of the work piece in known alignment with the firstpatterned image.
 14. The optical alignment system as in claim 13,wherein the work piece is a printed circuit board substrate.
 15. Amethod of optically aligning a substrate, comprising: establishing anoriginal coarse position of a substrate; creating original microscopicgrain images with one or more first cameras aimed at multiple regions ofthe substrate; establishing a new coarse position of the substrate afterrepositioning the substrate and obtaining new microscopic grain imagesof the multiple regions; and performing pattern comparison of theoriginal and new images of the respective regions until a pattern matchis recognized, the images recording unique microscopic non-uniformitiesinherent to the substrate, thereby finely locating the position of therepositioned substrate.
 16. The method as in claim 15, wherein thesubstrate is placed on a stage, the original microscopic grain imagesbeing obtained with at least one first camera viewing the multipleregions of the substrate through Windows in the stage.
 17. The method asin claim 16, wherein the repositioned substrate is a reversed substrate,the new microscopic grain images being obtained from at least one othercamera located on the opposite side of the substrate from the at leastone first camera and in known positional relationship to the at leastone first camera.
 18. The method as in claim 15, wherein the multipleregions comprise a first and second region on a first side of thesubstrate.
 19. The method as in claim 15, further comprising recording afirst patterned image on the substrate; and, after repositioning thesubstrate, recording a second patterned image on the repositionedsubstrate in known alignment with the first patterned image.
 20. Themethod as in claim 19, wherein the substrate is a printed circuit boardsubstrate.
 21. A method of aligning a reversible work piece, comprising:placing the work piece in an obverse position onto a stage; viewing andstoring microscopic grain images of first and second sites of a firstside of the work piece through the stage by one or more stage cameras;reversing the work piece on the stage; viewing and storing microscopicgrain images of at approximate locations of the first and second sitesby an area camera; computing a relative position of the reversed workpiece by performing a pattern match of the images obtained by the areacamera with the images obtained by the one or more stage cameras. 22.The method as in claim 21, further comprising: viewing and storing animage of a first corner of the work piece when placed on the stage inboth the obverse and reverse positions; and using the corner images toprovide a coarse alignment sufficient to compute approximate locationsof the first and second sites for viewing by the area camera.
 23. Themethod as in claim 21, further comprising recording a first patternedimage on a first side of the work piece; and after reversing the workpiece, recording a second patterned image on the a second side of thework piece in known alignment with the first patterned image.
 24. Themethod as in claim 23, wherein the work piece is a printed circuit boardsubstrate.